As a signal processing apparatus for canceling echoes caused by received signals propagating through spatial acoustic paths in a system using plural received signals and a single or plural transmission signals, the apparatus of a linear coupled type is disclosed in Non-Patent Literature 1. A block diagram of a multichannel signal processing apparatus of a linear coupled type in the case that the number of channels is two, namely, in the case of the signal processing apparatus having a stereo signal as a target, is shown in FIG. 17. According to Patent Literature 1, the apparatus of the linear coupled type has a problem that the coefficients have an indefinite number, namely a problem that adaptive filter coefficients converge to indefinite values other than values equal to the features of the echo paths (correct solutions). The filter coefficient value that has converged depends upon a cross correlation between the filter input signals, and thus, when the cross correlation is changed due to movement etc. of a far-end talker, the correct coefficient value also varies. A variation in the correct coefficient value caused by a change in the cross correlation means that the echo canceling capability is degraded even with no variation in the echo paths. Thus, the residual echoes are perceived, and thus, a speech quality is degraded.
In order to solve this problem, the signal processing method in which a single adaptive filter per channel estimates echoes caused by signals propagating from a single sound source in a plurality of paths by generating pseudo echoes (echo replicas) with adaptive filters corresponding one to one to mixed signals with one received signal as an input is disclosed in Patent Literature 2 A problem that the coefficients have an indefinite number does not exist in this signal processing method because one adaptive filter cancels the echo to be generated on one channel. As a result, the adaptive filter coefficients converge to optimum values that are uniquely determined. However, the Non-patent Literature 2 discloses evaluation results proving that the fact that the echo canceling capability is degraded when the parameters determined by the environment in use such as the locations of microphones receiving the taker's voice are not within a certain range. Hence, in order to use the cancellation apparatus in a wide variety of environments, a multichannel echo canceller of a linear coupled type has to be used.
Based upon this premise, Patent Literature 2 discloses the method capable of uniquely determining the adaptive filter coefficients by delaying the received signal of the multichannel echo canceller of the linear coupled type, thereby to generate the delayed signal, and continuously and mutually alternating this as a new received signal with the received signal. In this signal processing method, the number of conditionals used to calculate the adaptive filter coefficients is increased because of the introduction of the delayed received signals, whereby a problem that the solution, being the adaptive filter coefficient, becomes indefinite does not occur. As a result, the adaptive filter coefficients converge to optimum values that are uniquely determined. However, with the case of the method proposed in the Patent Literature 2, a movement of the acoustic image may often be perceived when the received signal and the delayed received signal are switched. The movement of the acoustic image is perceived as an unnatural sound because it seems as if the acoustic image had moved even though it does not move as a matter of fact, and hence, a subjective sound quality of the received signals is degraded. In order to solve this, the method of correcting the amplitudes of signals in both channels when the received signal and the delayed received signal are switched is disclosed in Patent Literature 3.
On the other hand, the method capable of uniquely determining the adaptive filter coefficients by applying a non-linear processing to the received signals in both channels instead of switching the received signal and the delayed received signal is disclosed in Non-patent Literature 3. However, Non-patent Literature 4 makes it clear that the methods disclosed in the Patent Literature 2, the Patent Literature 3, and the Non-patent Literature 3 provide a slow convergence rate, respectively, as compared with the multichannel echo canceller of the linear coupled type. It is shown in the Non-patent Literature 4 that the method disclosed in the Non-patent Literature 3 provides a yet slower convergence rate as compared with each of the method disclosed in the Patent Literature 2 and the method disclosed in the Patent Literature 3.